Equipments for providing such power transmission have been known for a long time and a certain number of patents have been filed in this field. Mention may be made, for example, of Swiss pat. No. 99,382 in the name Mr. Schoedelin, filed Sept. 31, 1921 and relating to "a trailer suitable for hitching to a motor vehicle, and when stationary for receiving and making use of the driving force of the vehicle engine". This patent describes a device including a running path constituted by a belt carried by two pulleys and driven by the rotary motion of the driving wheels of the vehicle which is capable of mounting thereon once the trailer carrying said device has been unhitched and the device placed on the ground. The power taken off by driving the belt is then transmitted to a building site type of machine mounted on said trailer.
French Pat. No. 932 494 in the name of Mr. Lafay, filed Aug. 21, 1946 also describes a "mechanical control device actuated by the wheels of a motor vehicle". This device comprises a stand resting on the ground and carrying two shafts mounted on bearings and each receiving two waisted cylinders or else two cylinders which are then connected in pairs by endless belts. By placing the wheels of a vehicle on this device, the wheels will cause the waisted cylinders or the belts to rotate and further belts are then used to transmit the motion to any type of machine.
British Pat. No. 352 954 in the name of Mr. James, filed Apr. 14, 1930 describes "improved means for the transmission of power from the driving or traction wheels of motor vehicles". This patent describes a device for raising the axle carrying the driving wheels of the vehicle and, once the wheels have been lifted off the ground, for pressing a take-off wheel against each drive wheel with the take-off wheels then being driven by friction and transmitting motion to a transmission assembly.
However, it may be observed that the above-mentioned equipments have been known for more than 40 to 60 years without giving rise, at any rate in France and at present, to any significant or major development of machinery using this principle of taking off force from the wheel of a motor vehicle.
The above devices and others which have been tested since suffer from numerous drawbacks, some of which even make them dangerous to use:
Firstly, mention may be made of the very poor stability of a vehicle when using a belt mounted on pulleys (as in the first two above-mentioned patents): in order to achieve drive without slipping, it is necessary for the wheels to bear heavily against a taut belt or for the contact area to be large if the belt is relatively slack. In the absence of the special means of the present invention, both of these conditions give rise to forces perpendicular to the direction of belt rotation and as a result the belts are very quickly dislodged from the pulleys, and even when the pulleys are fitted with side plates or catches, the belt is damaged or the support mechanism gives way or the vehicle skids which can be dangerous for personnel.
The above general problem is particularly acute on rear-wheel drive vehicles because it is impossible to align the devices and the vehicle wheels correctly. Lateral forces are then increased and nothing can stand up to them.
Another solution for avoiding the above drawback is to use friction wheels pressing directly against the driving wheels, as in the third above-mentioned patent. Although large lateral forces remain, the lateral forces are reduced because the friction wheels provide better tangential contact in the direction of rotation and mechanical take-off is more easily controlled than it can be using a belt which must be left free. However, in this case the contact surface is small and power is therefore lost by slipping, giving rise to heating and to tire wear.
Further, regardless of which of the above devices is used, it is necessary for the device to be chocked up when used on ground which is sloping, as is frequently the case out of doors. If the above-mentioned basic problem is not to be made worse, it is essential for the vehicle-and-device assembly to be exactly horizontal. This therefore requires long and tedious adjustment and chocking up.
Finally, it may also be underlined that most applications require rotation at a given and constant speed: this applies, for example, to producing electricity (generator set), to producing a flow of air (compressor), or to supplying water (pump), and it is necessary to ensure that drive occurs at constant speed regardless of the load applied by the driven machine. It is then difficult to adjust the engine of the motor vehicle accordingly.
The problem is to be able to recover the motion or power transmitted to the wheels of a vehicle by the engine, in perfect safety and with high efficiency, without danger for the user or for the equipment, while retaining the possibility of adjusting the speed of rotation to a constant number of revolutions per minute (rpm) regardless of the torque applied by the machine taking off the power.